Lens Numbers: Making Sense of them

Nikon 18-55mm

When I first got back into photography I realized I had forgotten a lot of what I had learned years ago. Digital photography was a whole new world for me and I jumped in without a lot of knowledge of my own, but with good advice from a local camera shop. When it came to lenses, I knew the basics, but didn’t truly understand what all the lens numbers meant. The numbers really aren’t all that complicated once you break them down, but you will need a little basic knowledge before we can go too far.

Lens Numbers: “mm”

When starting out in the world of digital single lens reflex (DSLR) cameras, most people start with a kit, or a package that contains a camera body and one or two lenses. That is how I started, and what I shot with for several years. The two most popular, Canon and Nikon, offer impressive cameras in kits for under $600 these days. The most popular lens is the 18-55mm lens. The 18-55 refers to the focal length of the lens which is “a calculation of an optical distance from the point where light rays converge to form a sharp image of an object to the digital sensor or 35mm film at the focal plane in the camera.” This quote is from a very good article about focal length on the Nikon USA website. What does that mean? It is a relative measurement of distance from front of the lens to the sensor element in the back of the camera. The shorter the distance, the lower the number.

So, an 18-55mm lens is a zoom lens that adjust from an 18mm focal length out to a 55mm focal length. This lens is popular because it works as a basic wide-angle lens at 18mm, allowing the photographer to gather in a wide view – think of a vacation shot in New York where you want to gather in as much of Time Square as you can. The lens also allows a zoom out to 55mm, which is a great focal length to take a portrait shot of your family. I know, almost every consumer kit DSLR is a “crop-sensor” camera and that changes the effective focal length of the lens, but that is a totally different post, and the concepts translate.

Other popular focal lengths are 55-200mm or 70-300mm zoom lenses. These take over where the 18-55mm lens stops. A zoom lens like this allows the photographer to zoom into the image. There is another type of lens, called a fixed focal length, or prime lens. A prime lens does not zoom so the focal length is a single number, 35mm, 50mm or 85mm are popular prime lens focal lengths. Everything is a tradeoff – when you give up the ability to zoom the lens, you gain a sharper lens. The first time I started shooting with a 50mm lens, I felt like I was using a much more expensive camera. For most photographers, the kit camera from Nikon or Canon is plenty of camera for them, and investing in better lenses is a better investment than a new camera body.

Lens Numbers: “1:3.5-5.6” or “1.4” or “1.8”

This is the maximum aperture, or the maximum opening, of the lens. Again, Nikon has a great article about this. In simple terms, the smaller the number the wider open the lens aperture can go to let in more light to the camera sensor or film. This aperture number,called an f-stop , is written f/3.5 or f/5.6. If you hear someone say that a lens is “fast”, they mean that this number is small. A “small” number is in the f/1.2 to f/2.8 range, and typically the smaller the number, the more expensive the lens. Why is there a range on some lenses? It goes back to the focal length. One of Nikon’s most popular kit lenses is an “18-55mm 1:3.5-5.6” lens. The focal length adjusts from 18-55mm and as the focal length adjusts so does the maximum aperture. This kit lens has a maximum aperture, or opening size, of f/3.5 to f/5.6 depending on where the focal length of the lens is set.

On a prime lens, or fixed focal length, a single number is stated. Most prime lenses are under f/2.0, meaning they are fast lenses that allow you to shoot in low light. The first time you shoot with a lens this fast, the shutter speed possible in lower light conditions will amaze you. These lenses are popular for portrait and wedding photographers. Our two favorites are a 50mm f/1.4 and an 85mm f/1.8 – the photos they produce are amazing.

On expensive zoom lenses you will see numbers like “70-200mm f/2.8.” This is a zoom lens with a variable focal length from 70 to 200mm and a fixed aperture at f/2.8. For a zoom lens, f/2.8 is considered fast and the Nikon and Canon versions of this lens are in the $2,500 range. This type of lens is expensive to manufacture because of the quality of components in the lens that are necessary to produce a lens that is so fast at a 200mm focal length. The quality of these lenses rival a prime lens – still not quite as good, but still excellent quality.

Lens Numbers: Filter Size or Lens Diameter (Zero with a slash through it, followed by number – “52mm”)

This is the opening or lens diameter at the front of the lens. This number is mostly used to know what size filter to buy for your lens. Yes, you should always have a filter on the front of your camera lens! A basic UV or even clear filter is probably the cheapest insurance you can buy. I hike with my camera and a lens filter has saved from being scratched my lens many times. Don’t buy a cheap filter or it will degrade your lens, buy a decent quality filter. You won’t regret it.

If you have never used a higher quality lens, you should try one out. Most photographers will not loan out lenses, just like your auto-mechanic won’t loan out tools, but there is an alternative. Many high-end camera stores will rent lenses. This is an opportunity for you to try out a higher quality lens for a day or two and see if you like it.

About David Fossler

Husband, Father, Pilot, Photographer, Blogger. I have been a pilot for over 25 years and taught or flown most modern general aviation aircraft up through the Cessna Caravan, Piper Meridian and Pilatus PC-12. My wife and I own a photography business and shoot with Nikon equipment. I grew up traveling - my first airline trip was at 6 weeks and I never looked back. Thank you for taking time to visit and read our blog.